add fallback 'nightly' category

After our discussion in the FFDW-DVD, fdroidclient should include a hardcoded, fallback category that fdroid nightly sets. Otherwise, nightly repos built with fdroidserver from Debian/bookworm or Ubuntu/22.04 LTS will no longer show up in Categories view. Here's where fdroid nightly sets the category: https://gitlab.com/fdroid/fdroidserver/-/blob/6124caf341b674f596b541978215b6683079e439/fdroidserver/nightly.py#L417

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information